Many digital cameras have autofocus capability. Autofocus may be fully automatic such that the camera identifies objects in the scene and focuses on the objects. In some cases, the camera may even decide which objects are more important than other objects and subsequently focus on the more important objects. Alternatively, autofocus may utilize user input specifying which portion or portions of the scene are of interest. Based thereupon, the autofocus function identifies objects within the portion(s) of the scene, specified by the user, and focuses the camera on such objects.
To achieve market adoption, the autofocus function must be reliable and fast such that every time a user captures an image, the camera quickly brings the desired portion, or portions, of the scene into focus. Preferably, the autofocus function is sufficiently fast that the user does not notice any delay between pressing the trigger button and image capture. The autofocus is particularly important for cameras having no means for manual focus, such as compact digital cameras and camera phones.
Many digital cameras use contrast autofocus, wherein the autofocus function adjusts the imaging objective to maximize contrast in at least a portion of the scene, thus bringing that portion of the scene into focus. More recently, phase-detection autofocus has gained popularity because it is faster than contrast autofocus. Phase-detection autofocus directly measures the degree of misfocus by comparing light passing through one portion of the imaging objective, e.g., the left portion, with light passing through another portion of the imaging objective, e.g., the right portion. Some digital single-lens reflex cameras include a dedicated phase-detection sensor in addition to the image sensor that captures images.
However, this solution is not feasible for more compact and/or less expensive cameras. Therefore, camera manufacturers are developing image sensors with on-chip phase detection, i.e., image sensors with integrated phase detection capability via the inclusion of phase-detection auto-focus (PDAF) pixels in the image sensor's pixel array.
FIG. 1 illustrates one exemplary image sensor 101 with PDAF pixels in an exemplary use scenario 190. Image sensor 101 is implemented in a digital camera 180 for imaging a scene 150. Digital camera 180 is, for example, a camera phone or a compact digital camera. Digital camera 180 utilizes the on-chip phase detection capability of image sensor 101 to focus on scene 150. When focused, digital camera 180 utilizes image sensor 101 to capture a focused image 120, instead of a defocused image 130, of scene 150.
Image sensor 101 has a pixel array 200A that includes at least one dual-diode PDAF pixel 200. FIG. 2 is a cross-sectional view of the dual-diode PDAF pixel 200 of pixel array 200A. Dual-diode PDAF pixel 200 includes photodiodes 211 and 212 having a common color filter 221 and microlens 230. Microlens 230 has an optical axis 231 centered between photodiodes 211 and 212. Photodiodes 211 and 212 have respective top surfaces 211T and 212T. Dual-diode PDAF pixel 200 may be viewed as including phase-detection pixels 200L and 200R, which include photodiode 211 and photodiode 212 respectively.
FIGS. 3A-3C are cross-sectional views of a PDAF imaging system 300 in which a lens 310 forms an image 352 of an off-axis object 350 at an image plane 312 proximate pixel array 200A. Lens 310 has an optical axis 310A that intersects pixel array 200A at a pixel-array center 200AC. Image 352 is at a radial distance 352R from optical axis 310A and pixel-array center 200AC. Image plane 312 and lens 310 are separated by an image distance 312Z.
FIGS. 3A-3C illustrate propagation of a chief ray 351(0), an upper marginal ray 351(1), and a lower marginal ray 351(−1). In the cross-sectional view of FIGS. 3A-3C, pixel array 200A includes a column of dual-diode PDAF pixels 200 of FIG. 2. In FIG. 3A, pixel array 200A is in front of image plane 312. In FIG. 3B, pixel array 200A is coplanar with image plane 312. In FIG. 3C, pixel array 200A is behind image plane 312.
FIGS. 3A-3C also include schematic pixel column responses 303 and 304, which represent response of, within a column of phase-detection pixels PDAF pixel 200, (a) left photodiodes 211 and (b) right photodiodes 212, respectively.
In FIG. 3A, pixel array 200A is behind image plane 312 such that image 352 is out of focus at pixel array 200A. Pixel array 200A is at a distance 311A from lens 310, which corresponds to a misfocus distance Δz=ΔzA>0 from image plane 312. Pixel column response 303A illustrates that a column of left phase-detection pixels detects one intensity peak 303A′ corresponding to upper marginal ray 351(1). Pixel column response 304A illustrates that a column of right phase-detection pixels detects one intensity peak 304A′ corresponding to lower marginal ray 351(−1). Intensity peak 304A′ is closer to optical axis 310A than intensity peak 303A′. On pixel array 200A, intensity peaks 303A′ and 304A′ are separated by a distance Δx=ΔxA>0.
In FIG. 3B, pixel array 200A is located at image plane 312 such that image 352 is in focus. Pixel array 200A is at a distance 311B from lens 310, which corresponds to a misfocus distance Δz=ΔzB=0 from image plane 312. Pixel column response 303B illustrates that a column of left phase-detection pixels detects one intensity peak 303B′ corresponding rays 351(−1,0,1) being incident on the same left-phase-detection pixel in the column. Pixel column response 304B illustrates that a column of right phase-detection pixels detects one intensity peak corresponding to rays 351(−1,0,1) being incident on the same right-phase-detection pixel in the column. On pixel array 200A, intensity peaks 303B and 304B′ are separated by a distance ΔX=ΔXB, which is illustrated as equal to zero in FIG. 3B
In FIG. 3C, pixel array 200A is in front of image plane 312 such that image 352 is out of focus at pixel array 200A. Pixel array 200A is at a distance 311C from lens 310, which corresponds to a misfocus distance Δz=ΔzC<0 from image plane 312. Pixel column response 303C illustrates that a column of left phase-detection pixels detects one intensity peak corresponding to upper marginal ray 351(1). Pixel column response 304C illustrates that a column of right phase-detection pixels detects one intensity peak corresponding to lower marginal ray 351(−1). Intensity peak 304C′ is further from optical axis 310A than is intensity peak 303C′. On pixel array 200A, intensity peaks 303C′ and 304C′ are separated by a distance Δx=ΔxC<0.
One indicator of the accuracy of phase-detection auto-focusing by image sensor 101, hereinafter “PDAF accuracy,” is how well the magnitude of Δx indicates the magnitude of misfocus Δz. Specifically, with reference to FIG. 3B, zero misfocus (Δz=0) should correspond to Δx=0. Hence, the smaller the magnitude of Δx is when Δz=0, the higher the PDAF accuracy.